As with most 404s, you don't need to do anything. That's assuming you didn't do anything in the first place to make the visitor think you have a data:image file. It's one of a handful of nonexistent files that perfectly law-abiding visitors will sometimes ask for. (There's a list somewhere, but I can't remember where.)
data:image is a way to store a tiny image, like an icon, as raw data instead of a separate image file. That is: you could use it on big images, but the data file would be huge. If you ever get bored, there are www sites where you can feed in an image and it will spit out the data:image code for you.